This application relates to similar subject matter as U.S. Ser. No. 10/339,376 filed Jan. 9, 2003, entitled “Servo Error Detecting Device for Optical Disk,” the entirety of which is incorporated herein by reference.
FIG. 1 (prior art) shows an example of the configuration of a conventional focus error detector in an optical disk device. This focus error detector utilizes operational amplifiers and comprises analog adders 200, 202, a subtractor 204, a coefficient multiplier 206, an offset adjustor 208 and an amplifier 210 connected as shown.
FIG. 2 shows light receiving regions A, B, C, D of an optical detector 212 of an optical pickup of an optical disk device for generating RF (radio frequency) signals based on light reflected off an optical disk and incident thereon. RF signals SA and SC obtained from light receiving regions A and C positioned along one of the diagonals are respectively input to the two input terminals of adder 200, and RF signals SB and SD obtained from light receiving regions B and D positioned along the other diagonal are respectively input to the two input terminals of adder 202. These RF signals SA, SB, SC, SD are typically voltage signals obtained by I-V conversion inside the optical pickup, with a reference voltage applied as a bias voltage on the pickup from the outside taken as a reference level.
Adder 200 takes the sum of the input signals SA and SC, and outputs a sum signal (SA+SC). The sum signal (SA+SC) is input to the positive side input terminal of subtractor 204. Adder 202 takes the sum of the input signals SB and SD, and outputs a sum signal (SB+SD). This sum signal (SB+SD) is multiplied by coefficient Gb by coefficient multiplier 206, and the obtained signal is input to the negative side input terminal of subtractor 204. Multiplier Gb serves to correct the balance of the comma aberration using an astigmatic aberration method.
Subtractor 204 takes the difference between the two input signals, and outputs a difference signal {(SA+SC)−Gb(SB+SD)}. This difference signal is a signal that represents the focusing error. Typically, for example, an offset portion Offset is cancelled at an offset adjustor 208, and the signal is Ga-fold amplified at an amplifier 210. Then, through an anti-aliasing filter or a low-pass filter (not shown in the figure), the signal is fed as a focus error signal FE to an A/D converter of the next stage (not shown in the figure).
The formula for performing the operation to generate focus error signal FE at the focus error detector can be represented by the following Equation (1):FE=Ga{(SA+SC)−Gb(SB+SD)−Offset}  (1)
FIG. 3 (prior art) shows an example of the configuration of a conventional tracking error detector in a push-pull system. This tracking error detector utilizes operational amplifier circuits and comprises analog adders 214, 216, a subtractor 218, an amplifier 220, and an offset adjustor 222. Adder 214 takes the sum of RF signals SA and SD from left-side light-receiving regions A and D of the optical detector, and outputs it as a sum signal (SA+SD). Adder 216 takes the sum of RF signals SB and SC from right-side light-receiving regions B and C, and outputs it as a sum signal (SB+SC). Subtractor 218 takes the difference between the two signals and outputs a difference signal {(SA+SD)−(SB+SC)} which represents the tracking error. Usually, amplifier 220 amplifies with a Gc-fold gain; offset adjustor 222 cancels offset portion Offset; and the signal is fed through an anti-aliasing filter or low-pass filter (not shown in the figure) to an A/D converter (not shown in the figure) of the next stage as a tracking error signal TE.
The formula for performing the operation to generate tracking error signal TE at the tracking error detector can be represented by the following Equation (2):TE=Gc{(SA+SD)−(SB+SD)}−Offset  (2)
As explained above, a conventional servo error detector is made of a combination of various analog circuits comprising operational amplifiers. In this case, one of the problems is that it lacks general applicability. In an optical disk device, the state and level of the output signal of the optical detector depend on the specifications of the optical pickup and optical disk. Consequently, it is necessary to amend the servo error detector and to change the gain and offset values corresponding to these specifications. For example, when offset adjustor 208 and amplifier 210 are swapped with each other (with respect to the front/rear relationship) in the focus error detector, while the difference in the operation formula relates to whether the offset value Offset is outside or within the parentheses, individual circuit configurations take place on the IC (integrated circuit) level.
Also, the astigmatic aberration method is an example of focus error detection. There are also three or four types of focus error detecting systems. As far as detection of the tracking error is concerned, the aforementioned push-pull system is merely an example, there are also other systems that may be used. Consequently, it is necessary to prepare a servo error detector for each system or each set of specifications.
Also, the circuit configuration becomes more complicated. This is also a problem. For example, for a device for reproduction of a RW (re-write phase-change type) optical disk, which has a reflectivity of about ¼ or lower that of a CD-ROM or other conventional stamped disk, the level of the RF signal output from the optical detector is rather low. Consequently, one has to significantly increase the gain of the servo error detector to compensate for this. At this point, in a conventional servo error detector, an offset canceling circuit of an operational amplifier has to be set for each section of the operation, and the circuit configuration becomes complicated with respect to its adjustment and setting. That is, the configuration cannot be made as simple as indicated by said operation equations (1) and (2).
In addition, there is also problem in the precision of the servo error signal. For an optical disk device, the precision of the servo depends on the precision of the servo error signal. For example, in a tracking servo, when the optical pickup jumps from the track being tracked to another track, if the precision of the tracking error signal is not high, a long time is required from application of the tracking servo near the target track to the track ON state, and it is hard to perform the tracking correctly.
FIGS. 4A and 4B are diagrams schematically illustrating the waveforms of RF signals SA, SB, SC, SD obtained using light receiving regions A, B, C, D of the optical detector in the case of track jumping. For the waveforms of the various RF signals, sinusoidal level variation Ste appears on the bottom. This is because the beam spot of the laser beam radiated from the optical pickup on the signal recording surface of the optical disk alternately traverses the track portion (pit) and the mirror portion between tracks (the flat portion without the pit). When the beam spot traverses the track portion, the intensity modulation or diffraction due to the pit becomes maximized (as a result, the optical intensity of the reflected optical beam becomes minimum), and when beam spot SP traverses the mirror portion, the intensity modulation becomes a minimum (as a result, the optical intensity of the reflected light beam becomes maximum). Usually, even when the beam spot is at the center of the mirror portion, it also overlaps a portion of the adjacent track, and intensity modulation takes place in the overlapped portion. Consequently, in the RF signal, the maximum point on the bottom level does not become as high as the top level, and even near the maximum point, the radio frequency modulation component lasts. In the push-pull system, such tracking modulation components Ste have the same phase for RF signals SA and SD and RF signals SB and SC, and they have the opposite phase for RF signals (SA, SD) and RF signals (SB, SC).
In a conventional tracking error detector (FIG. 3), by performing the desired push-pull operation using analog operation circuits 214, 216, 218 for RF signals SA, SB, SC, SD obtained from light receiving regions A, B, C, D of the optical detector, it is possible to obtain a tracking error signal TE that represents the position error of the beam spot in the radial direction. However, in both low-speed reproduction and high-speed reproduction, after passing through the low-pass filter, tracking error signal TE is significantly attenuated (by about half), and the SN (signal-to-noise) characteristics degrade. This is a problem (see FIGS. 5 and 6). This is due to the fact that the top envelope component of the RF signal is nearly flat. Also, in the case of low-speed reproduction, the frequency of the RF signal and the frequency of the tracking signal component are rather close to each other, and it is hard to separate the two signals with a low-pass filter. As shown in FIG. 5, RF signal component RFn may be easily left in the tracking error signal TE.
For a CD (Compact Disc), DVD (Digital Versatile Disk), or other general optical disk, EFM (Eight to Fourteen Modulation) or EFM Plus is adopted as the modulation system of data recorded on the tracks, and the pit length is limited in the range of 3T–11T (T represents the length of 1 bit) so that the [2,10]RLL (Run-Length Limited) coding condition is met, that is, the length of continuous “0” is at least 2 and less then 11. Consequently, for a CD as an example, the frequency of the channel clock in the case of x1 reproduction is 4.32 MHz. On the other hand, the maximum frequency of RF signal in the case of 3T/3T is 720 kHz, and it decreases to about 196 kHz in the case of 11T/11T. In order to perform tracking servo correctly, it is preferred that such RF signal component in the tracking error signal be so small so that it may be ignored.
However, in a conventional tracking error detector, it is hard to separate or cut an RF signal component at about 196 kHz with respect to the tracking error signal TE obtained from tracking modulation component Ste usually at tens of kHz, even after passing a low-pass filter.
Also, when conversion is performed to a digital signal, in order to fit the input range of the A/D converter of the next stage, the amplitude of tracking error signal TE is increased by means of operation circuits 214, 216, 218 and amplifier 220. However, since the RF signal component is also amplified at the same time, the gain cannot be increased sufficiently. Also, since the RF signal component is mixed, the precision of the digital signal is low.
On the other hand, in high-speed reproduction, that is, x30 or a higher speed for a CD and x6 or a higher speed for a DVD, the bandwidth of the RF signal becomes 10 MHz or larger, and when the bandwidth of operation circuits 214, 216, 218 is only several MHz, these operation circuits act as a low-pass filter. As a result, not only is the RF signal component attenuated, but the tracking modulation component Ste as the base of tracking error signal TE is attenuated to ½, that is, to half the value. As a result, the SN decreases by 6 dB.
For a conventional focus error detector (FIG. 1), too, the focus error signal precision is not high, and the SN ratio is low. This is undesirable. The same problem exists also for various other types of servo error detecting systems.
There is a need to provide a servo error detector usable for an optical disk device that can realize higher precision, higher stability, lower cost, and higher stability. There is also a need to provide a servo error detector usable for an optical disk device that has good general applicability.